1. Technical Field
The present invention relates to surface acoustic wave resonators used for telecommunications devices of high frequency bands such as mobile phones.
2. Background Art
There is a rising demand in recent years for surface acoustic wave resonators having filtering characteristics over wide frequency bands. In order to produce wide frequency band filters, it is generally necessary to choose substrates made of a material having a large electromechanical coupling coefficient.
LiNbO3 substrates used conventionally are effective for widening frequency bands of the surface acoustic wave filters because of the large electromechanical coupling coefficient. In general, however, LiNbO3 substrates degrade the in-band frequency characteristics due to spurious responses caused by resonance in the transverse mode peculiar to them.
FIG. 8 shows a plan view of a conventional surface acoustic wave resonator provided with normal type electrodes. In normal type electrode 3 having comb electrodes 1 and reflectors 2 as shown in FIG. 8, there occur spurious responses attributed to the resonance in the transverse mode. FIG. 9A and FIG. 9B represent a characteristic of a typical case stated above. That is, FIG. 9A shows a pass-band characteristic, which plots insertion loss versus frequency of the surface acoustic wave resonator of FIG. 8, and FIG. 9B shows a part of the pass-band characteristic of the surface acoustic wave resonator, wherein domain 9b encircled by a dotted line in FIG. 9A is enlarged.
As can be seen from FIG. 9A and FIG. 9B, there occur spurious responses between resonance point 9c and anti-resonance point 9d. There is a technique generally known to suppress the spurious responses caused by the resonance in the transverse mode, which is to apodize the surface acoustic wave resonator as illustrated in FIG. 10. That is, FIG. 10 shows a plan view of a conventional surface acoustic wave resonator of different type from that of FIG. 8. This surface acoustic wave resonator is formed into a so-called apodized configuration, in which overlapping lengths of interdigitated electrode fingers of two comb electrodes 1 are varied as shown in FIG. 10 to obtain a desired frequency characteristic.
Patent references 1 and 2 are examples of the prior art documents known to be related to the above technique.
Although the apodization is effective to suppress the resonance in the transverse mode of comb electrodes 1 having a small number of electrode fingers as shown in FIG. 10, it is not feasible to suppress spurious responses if comb electrodes 1 have a large number of electrode fingers as is the case of another conventional surface acoustic wave resonator shown in FIG. 11 since it bears a high amplitude of excitation in the transverse mode.
FIG. 12A shows a pass-band characteristic of a surface acoustic wave resonator provided with comb electrodes having 150 apodized electrode fingers, and FIG. 12B shows a part of the pass-band characteristic of the surface acoustic wave resonator, wherein domain 12b encircled by a dotted line in FIG. 12A is enlarged.
It is apparent from the pass-band characteristic shown in FIG. 12A and FIG. 12B that the spurious responses by the resonance in the transverse mode are reduced as compared to that of FIG. 9A and FIG. 9B.
FIG. 13A shows a pass-band characteristic of another surface acoustic wave resonator provided with comb electrodes having 300 apodized electrode fingers, and FIG. 13B shows a part of the pass-band characteristic of the surface acoustic wave resonator, wherein domain 13b encircled by a dotted line in FIG. 13A is enlarged.
The surface acoustic wave resonator featuring the pass-band characteristic shown in FIG. 13A and FIG. 13B has such a structure that comb electrodes are formed on a LiNbO3 substrate, and surfaces of the comb electrodes and the LiNbO3 substrate are covered with a thin film of silicon dioxide (hereinafter referred to as “SiO2”). The substrate used here is a rotated Y-cut substrate having a cut angle of 5 degrees. Although adoption of this structure can help widen a bandwidth and improve a temperature characteristic of the surface acoustic wave resonator, it is not effective to suppress the spurious responses attributed to the resonance in the transverse mode when the number of electrode fingers of the comb electrodes is increased to about 300, for instance. When such a surface acoustic wave resonator is used to compose a ladder type filter, it is necessary in design to increase a capacitance of the surface acoustic wave resonator. However, this gives rise to a problem that spurious responses are liable to occur when the number of electrode fingers of comb electrodes 1 is increased to obtain a large capacitance.    [Patent Document 1] Japanese Patent Unexamined Publication, No. 1990-11012    [Patent Document 2] Japanese Patent Unexamined Publication, No. 1994-85602